1. FIELD OF THE INVENTION
The present invention relates to a piezoelectric motor which generates a rotary or linear driving motion by using a piezoelectric substance as its driving element.
2. DESCRIPTION OF THE PRIOR ART
In recent years, a piezoelectric motor which produces a rotary or running motion by exciting various ultrasonic vibrations using an electro-mechanical transducer such as a piezoelectric ceramic or the like has been drawing attention because of its having a high energy density and other advantageous features.
Firstly, a description is made of the conventional piezoelectric motor utilizing these ultrasonic vibrations in reference to the drawings.
In the conventional piezoelectric motor reported in the Nikkei Mechanical (Feb. 28, 1983), an annular piezoelectric element is bonded to the surface of a thick metal ring or the like into one body to be employed as a stator.
When AC signals which are phase-shifted from each other, in the same way as in two-phase or three-phase motor, are applied to the conventional piezoelectric motor constituted as described above, expansion and contraction take place in the circumferential direction of the piezoelectric element. A bending travelling wave is generated on the above-mentioned stator.
A piezoelectric motor which utilizes a Rayleigh wave has also been proposed. This wave propagates in the vicinity of the surface of the substance having both components of the longitudinal wave and the transverse wave. FIG. 1 is a magnified view of a small part showing a state of contact of the stator 100 of the conventional piezoelectric motor with a slider 200 in contact with the surface thereof. The motion of material particles attending the bending wave have an elliptic locus when observed at a point A on the surface of an elastic body of the stator 100; the point A depicts an elliptic locus of "W" of major axis and 2u of minor axis. At a peak where the elastic body comes in contact with the slider 200, the point A has a velocity of V=2.pi.f in the negative direction of the X-axis. Resultantly, the slider 200 is driven at a velocity of V in the direction opposite to that of travel of the wave by a friction force with the elastic body. Thus, by depicting the elliptic locus as a driving force on the surface of the elastic body, the conventional piezoelectric motor drives the contacting slider 200 to rotate. Such a driving principle is clarified also in the Japan Patent Publication No. Sho 58-32518, and a concept of use of traveling wave consisting of the longitudinal wave and the transverse wave is disclosed such that, by exciting the longitudinal vibration and the transverse vibration generated at a frictional contact part by the same frequencies with the phase shifted from each other, a force in the transverse direction is generated to move an object in contact therewith.
The conventional configuration as mentioned above has the following various problems.
(1) The stress required for obtaining the vibration mode of a driving principle shows a maximum value at the surface of the stator. In a case of the stator of 3 mm in thickness, the vertical stress becomes about 2,000 kg/mm.sup.2, and the power required for this stress becomes about 100-1000 times the theoretical value of the bimorph.
(2) Because the neutral point of vibration is located within an elastic body provided in the stator such as metal or the like, the piezoelectric substance as an electro-mechanical transducer does not become an efficient drive, but becomes an inefficient maximum displacement position drive. According to this driving principle, 5/8 or more of the total energy becomes useless, even if only the piezoelectric body as a driving source is concerned.
(3) Because the driving force is picked up from a very small amplitude of about 0.25 .mu.m or less, the slider in uniform contacts with both maximum displacement point and minimum displacement point having different occurrence speeds and the directions thereof, respectively, and the slider velocity becomes a slow speed close to the integrated value. For this reason, a large power of as much as 10-100 times that of the magnetic motor is required to obtain a practical rotation speed, torque and the like.
(4) In the conventional piezoelectric motor, the driving electrode is divided into two pairs, A and B. The effective drive area never exceeds 50%. Therefore only a weak vibration, never exceeding 50% could be obtained by this electrode.